1. Field of the Invention
The invention relates to an actuating device for automatically actuating a manual transmission.
2. Discussion of the Prior Art
German reference DE 43 09 027 A1 has already disclosed an actuating cylinder for actuating a shifting shaft. The actuating cylinder has a first hydraulic cylinder and a second hydraulic cylinder. The first hydraulic cylinder is a double-acting cylinder which acts on an actuating piston to which the shifting shaft is firmly connected. The shifting shaft can be driven in a translatory fashion by the first hydraulic cylinder. The second hydraulic cylinder is connected via a driver element, which is connected in turn to the actuating piston in a fixed but axially displaceable fashion. A translatory movement of the piston of the second hydraulic cylinder, which is connected to the driver element via spiral teeth, is converted by the driver element into a rotary movement. The shifting shaft can therefore be driven in a rotary fashion by the second hydraulic cylinder. Given an appropriate drive, the shifting shaft can be automatically actuated by the actuating cylinder. Sensors are provided for detecting the paths of the actuating movements which can be initiated by the hydraulic cylinders. For example, it is possible to provide linear potentiometers as sensors. It is preferable to use sensors which operate without contact and which comprise a permanent annular magnet attached to the actuating piston, and a coil element, attached to the cylindrical outer wall of the housing, with primary and secondary windings. The absolute position of the shifting shaft is detected by the sensors. It is disadvantageous that because of the sliding-action contact the linear potentiometers have a short service life, and that the contactless magnetic sensors are very expensive.
European reference EP 599 511 A1 discloses a further device for automatically actuating a transmission. This device comprises a shifting finger which can be deflected via assigned drives into an X-direction and a Y-direction perpendicular thereto. The absolute position of the shifting finger in the respective direction of movement or coordinates is detected in each case by assigned potentiometers. The potentiometers are used to detect the transmission position.
Furthermore, the current which is applied to the drives during driving is sensed. If the sensed values deviate from predetermined values to more than a predetermined extent, the deviation of desired and actual values is reduced or corrected by running through a control loop.
In the case of this device, as well, potentiometers which have a short service life because of the sliding-action contact required are provided for detecting position.
German reference DE 36 13 800 C2 discloses an emergency actuation device which has a mechanical switch to be actuated by the driver. The switch has a plurality of actuating positions, the engagement of a predetermined driving gear being permitted by actuating the switch in a predetermined way. It is disadvantageous that the driver must, on the one hand, know how the emergency switch is to be actuated in the event of failure of the actuating device. Thus, in the stressful situation of a malfunction, the driver must have at his fingertips the required actuating routine in which the switch is to be actuated for the emergency actuation. This is very inconvenient for the driver. Furthermore, only a very restricted driving operation is possible, since only a predetermined driving gear can be engaged by the emergency switch.
German reference DE 44 33 825 C2 discloses a sensor for detecting a relative movement that operates in a contactless fashion.